1. Field of the Invention
The present invention relates to the vertical blanking interval (VBI) of TV signals, and more particularly, to an apparatus and method for detecting the vertical blanking interval.
2. Description of the Prior Art
The vertical blanking interval (VBI) is a blank interval reserved in a TV signal for the attachment of all kinds of user information. FIG. 1 shows the positions of scan lines for the VBI in different TV specifications. In the National Television System Committee (NTSC) system, each video frame has 525 scan lines; in the Phase Alternating Line (PAL) system, each video frame has 625 scan lines. FIG. 1 illustrates the scan line numbers for Closed Caption (CC), Copy Generation Management System (CGMS), Wildscreen Signaling (WSS), Video Programming System (VPS), and Teletext (TTX) 625B.
FIG. 2 is a schematic diagram of a typical VBI signal. As shown, the VBI signal contained in a scan line comprises the following portions: Hsync signal, color burst signal, clock run-in signal, frame code and data. Different VBI types correspond to different clock run-in signals and frame codes. A conventional VBI decoder is configured according to scan lines positions for a VBI signal within the TV signal. For example, VBI decoding is set to start when the scan line at a certain position is received. The VBI decoding first digitizes the received signals in reference to a preset constant level, for example, the DC voltage level, where the received signal is taken as 1 if its level is above the preset level, and 0 if its level is below the preset level. Next, the digitized signals are subject to slicing and parsing to complete the decoding.
However, the prior art is unable to identify whether the received signal is a VBI signal and hence unable to filter non-VBI noises. In addition, different reference levels needs to be set for different TV specifications, and also the level value should vary under different operating conditions. Hence using a constant reference level for signal digitizing lacks flexibility and accuracy.